Functional analysis of the murine chromosomal segment, homologous to the human region deleted in DiGeorge syndrome, identified a putative transcription factor, Tbx1, required for the development of the pharyngeal apparatus. Homozygous mutation of Tbx1 leads to severe disruption of pharyngeal arches 1-2, and absence of pharyngeal arches 3-6. The pharynx lacks the characteristic segmented pattern to assume a tube-like morphology. On the basis of the mutant phenotype, we hypothesize that Tbx1 has multiple roles in the development of the pharyngeal apparatus. This hypothesis cannot be addressed with the current germ-line mutation model because the early requirement of Thx 1 prevents us from testing gene function later in development. To address our hypothesis we propose to develop and test an in vivo conditional system for positive and negative regulation of Tbx1 expression. First we will construct and test the system, and subsequently, we will establish the temporal requirement of Tbx1 expression, and the role of Thx 1 after the initial pharyngeal endoderm patterning. Tbx1 mutants present one of the most dramatic and yet specific pharyngeal phenotypes so far reported. The strategy proposed here is intended to address a broad range of questions concerning the development of the pharyngeal apparatus. This is because the tools generated will allow us to modulate the development of this complex structure. Developmental problems of the pharyngeal apparatus underlie many birth defects, including relatively common craniofacial and cardiovascular abnormalities.